CVE-2026-89546: SUNRPC: close backchannel before destroying callback service

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: close backchannel before destroying callback service

A backchannel receive can complete a request while the NFS callback
service is being torn down.  xprt_complete_bc_request() removes the
request from bc_pa_list, drops bc_alloc_count, marks the request in use,
and then asks xprt_enqueue_bc_request() to hand it to the callback
service.

If teardown has already cleared xprt->bc_serv, xprt_enqueue_bc_request()
currently returns without enqueueing or freeing the committed request.
The xprt_get() taken on entry is leaked as well.  If the producer wins
the race before bc_serv is cleared, it can also enqueue onto sv_cb_list
after nfs_callback_down() has stopped the callback threads, leaving the
request linked to a svc_serv that is about to be freed.

Close the producer side before callback threads are stopped.  Add
xprt_svc_shutdown_bc() to clear xprt->bc_serv under bc_pa_lock, and call
it on callback shutdown and callback-start failure before stopping the
service threads.  Requests that lose the NULL transition in
xprt_enqueue_bc_request() are released through the normal backchannel
free path after balancing bc_slot_count.  Finally, drain any remaining
sv_cb_list requests after the callback threads have stopped and before
svc_destroy() frees the service.

The Linux kernel CVE team has assigned CVE-2026-89546 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.19 with commit 441244d4273a8037b265fd254dfdaca5fa736ee2 and fixed in 7.2.4 with commit 6debde9e3e6ae21dcca75837b4247b13ca4ea2b8
	Issue introduced in 6.19 with commit 441244d4273a8037b265fd254dfdaca5fa736ee2 and fixed in 7.3-rc1 with commit 3674f780f47d2906b5a0f7199b66973067bdfeca

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89546
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/nfs/callback.c
	include/linux/sunrpc/bc_xprt.h
	net/sunrpc/backchannel_rqst.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/6debde9e3e6ae21dcca75837b4247b13ca4ea2b8
	https://git.kernel.org/stable/c/3674f780f47d2906b5a0f7199b66973067bdfeca